Add mappings for UUID in PostGISDialect

Here's a tiny patch that adds some UUID mappings for Postgis. UUID columns weren't showing up for Postgis stores in Geoserver's parameterized sql view dialog (use select '00000000-0000-0000-0000-000000000000'::uuid as testcol; to test). After adding these mappings it works.

http://jira.codehaus.org/browse/GEOT-4414

Will backport to 9.x...